Color devices include input devices (e.g., scanners, still cameras, video cameras), output devices (e.g., printers), and display devices (e.g., Cathode Ray Tube (CRT) monitors, LCD display panels, television (TV) sets, high definition television sets). The operation of a color device is typically influenced by a number of operating conditions. For example, the scanned image of a scanner may be influenced by the age of the light source of the scanner; the appearance of the output of a printer under a standard viewing condition may be influenced by the characteristic of print media (e.g., paper) and ink level, as well as environment conditions such as humidity, temperature and pressure. The appearance of the image on a display device can be influenced by the background color, the intensity and color of the reflected ambient light.
Many methods to adjust the operation of color devices have been developed to account for the influence of environment conditions. For example, a television set can have a light sensor to automatically adjust the brightness level of the television set according to intensity of the ambient light detected by the light sensor. When the ambient light is bright (e.g., in the day time), the brightness of the television set is automatically increased; and when the ambient light is dim (e.g., at night), the brightness of the television set is automatically decreased.
Due to the variation of the ambient illumination and the observation conditions, the color on a screen may be perceived differently from one viewing condition to another. For example, a gray color may be perceived to be neutral (without color cast) in an office environment but pinkish in the daylight ambient illumination. A more complex situation may arise in the presence of mixed illuminants, for example when the fluorescent light in an office is mixed with the daylight coming through the windows. The viewing condition of a portable computer may change frequently, since the portable computer may be frequently moved to various locations of different environment conditions.
To account for the ambient illumination, some display systems (e.g., as described in U.S. Pat. Nos. 5,670,985 and 5,726,672) compensate the output of a device to offset the reflected ambient illumination. After the user determines the color and intensity of the reflected ambient illumination, the processor uses the tristimulus values of the ambient illumination to determine the bias setting of the device to compensate all outputs generated by the output device for the ambient illumination reflected from the device. The reflected ambient light is subtracted from the displayed color so that the resulting color on the display, under the influence of the ambient light, is the same (having the same tristimulus values) as the color displayed without the influence of the ambient light. In such an approach, the color correction is based on the instrumental measurements (e.g., tristimulus values) of the color. The perception of color from the user and the adaptation of the observer to the ambient illumination and the background colors in the surrounding environment are not considered. However, as the ambient light changes, the adaptation of the observer to the environment causes the observer to change the perception of the color on the screen, even if the color on the screen is corrected to remain colorimetrically the same according to the instrumental measurements. Thus, user experiences show that color adjusted (corrected) in this way may be perceived as having a hue shift (a color cast); and, such an approach may not be the preferred solution from the point of view of perceived color for an observer.
Some systems allow users to select a white point (e.g., along a black body curve, which represents the color of the light emitted by a theoretical “black body” at different absolute temperatures) and the target gamma, a well known parameter that characterizes the nonlinear intensity correction for CRT signals. However, adjusting the white point temperature and the target gamma may not be enough to compensate the influence of the ambient light. For example, when the screen color appears to have a hue shift of colors other than greenish or pinkish (purplish), the adjustment of the white point temperature may not be able to correct the color and remove the hue shift. If the display is calibrated for daylight illumination, the display may look greenish under office fluorescent illuminant; and, there is no way to correct such a hue shift based on the white point temperature adjustment; this may cause frustration for the user in not being able to adjust the color of the display to its preferences with the limited resources available for changing only the white point temperature of the display.
A color correction operation typically includes gamma correction, white point correction, color matching (or mapping), and others. It is understood that, in this application, the typical adjustment of the brightness level of a display device, which may be performed manually by a user through a control button or automatically according to the measurement of a light sensor, is not considered a color correction operation.